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• Unlike C++ or Java, no boolean type (in C89/C90) • in C99, bool type available (use stdbool.h)
6.087 Lecture 3 – January 13, 2010 Review Blocks and Compound Statements Control Flow Conditional Statements Loops Functions Modular Programming Variable Scope Static Variables Register Variables 1 Review: Definitions • Variable - name/reference to a stored value (usually in memory) • Data type - determines the size of a variable in memory, what values it can take on, what operations are allowed • Operator - an operation performed using 1-3 variables • Expression - combination of literal values/variables and operators/functions 1 Review: Data types • Various sizes ( char , short , long , float , double ) • Numeric types - signed / unsigned • Implementation - little or big endian • Careful mixing and converting (casting) types 2 Review: Operators • Unary, binary, ternary (1-3 arguments) • Arithmetic operators, relational operators, binary (bitwise and logical) operators, assignment operators, etc. • Conditional expressions • Order of evaluation (precedence, direction) 3 6.087 Lecture 3 – January 13, 2010 Review Blocks and Compound Statements Control Flow Conditional Statements Loops Functions Modular Programming Variable Scope Static Variables Register Variables 4 Blocks and compound statements • A simple statement ends in a semicolon: z = foo(x+y); • Consider the multiple statements: temp = x+y ; z = foo ( temp ) ; • Curly braces – combine into compound statement/block 4 Blocks • Block can substitute for simple statement • Compiled as a single unit Variables can be declared inside • { i n t temp = x+y ; z = foo ( temp ) ; } • Block can be empty {} No semicolon at end • 5 Nested blocks Blocks nested inside each other • { i n t temp = x+y ; z = foo ( temp ) ; { f l o a t temp2 = x ∗y ; z += bar ( temp2 ) ; } } 6 6.087 Lecture 3 – January 13, 2010 Review Blocks and Compound Statements Control Flow Conditional Statements Loops Functions Modular Programming Variable Scope Static Variables Register Variables 7 Control conditions • Unlike C++ or Java, no boolean type (in C89/C90) • in C99, bool type available (use stdbool.h) • Condition is an expression (or series of expressions) e.g. n < 3 or x < y || z < y Expression is non-zero condition true • ⇒ • Expression must be numeric (or a pointer) const char s t r [ ] = "some text" ; i f ( s t r ) / ∗ s t r i n g i s not n u l l ∗ / retu rn 0; 7 [...]... accessed/modified in any function 29 Divide and conquer • Break down problem into simpler sub-problems • Consider iteration and recursion • How can we implement gcd(a,b) recursively? • Minimize transfer of state between functions • Writing pseudocode first can help 30 6.087 Lecture 3 – January 13, 2010 Review Blocks and Compound Statements Control Flow Conditional Statements Loops Functions Modular Programming... v a l ; i ++) { i f ( a % i ) / ∗ i not d i v i s o r of a ∗ / continue ; i f ( b % i == 0 ) / ∗ i i s d i v i s o r o f both a and b ∗ / r e t = i ; } r e t u r n r e t ; } 24 6.087 Lecture 3 – January 13, 2010 Review Blocks and Compound Statements Control Flow Conditional Statements Loops Functions Modular Programming Variable Scope Static Variables Register Variables 25 Functions • Already seen some... variables (x,y) } return gcd and state variables (x,y) [Menezes, A J., et al Handbook of Applied Cryptography CRC Press, 1996.] © CRC Press All rights reserved This content is excluded from our Creative Commons license For more information, see http://ocw.mit.edu/fairuse 28 Returning multiple values • Extended Euclidean algorithm returns gcd, and two other state variables, x and y • Functions only return... until one is met; inner statement then executed • If multiple conditions true, only first executed • Equivalent to nested if statements 11 Nesting if statements i f ( x % 4 == 0 ) i f ( x % 2 == 0 ) y = 2; else y = 1; To which if statement does the else keyword belong? 12 Nesting if statements To associate else with outer if statement: use braces i f ( x % 4 == 0 ) { i f ( x % 2 == 0 ) y = 2; } else y... program using functions? 25 Divide and conquer • Conceptualize how a program can be broken into smaller parts • Let’s design a program to solve linear Diophantine equation (ax + by = c,x, y: integers): get a, b, c from command line compute g = gcd(a,b) if (c is not a multiple of the gcd) no solutions exist; exit run Extended Euclidean algorithm on a, b rescale x and y output by (c/g) print solution... statement • Contents of switch statement a block • Case labels: different entry points into block • Similar to labels used with goto keyword (next lecture ) 16 Loop statements • The while loop • The for loop • The do-while loop • The break and continue keywords 17 The while loop while ( / ∗ c o n d i t i o n ∗ / ) / ∗ l o o p body ∗ / • Simplest loop structure – evaluate body as long as condition is...Conditional statements • The if statement • The switch statement 8 The if statement i f ( x % 2) y += x / 2 ; • Evaluate condition if (x % 2 == 0) • If true, evaluate inner statement y += x/2; • Otherwise, do nothing... o r i a l ( int n) { int j = 1; int i = 1; /∗ i n i t i a l i z a t i o n ∗/ while ( i